1. Field of the Invention
The present invention relates generally to the field of orthodontics. More specifically, the present invention discloses an orthodontic appliance using a resilient vesica filled with a fluid to position teeth.
2. Statement of the Problem
When reduced to its simplest principles, orthodontic correction involves the steps of placing spring devices adjacent to individual teeth or groups of teeth according to a pre-determined treatment plan; loading those springs with potential energy; mechanically directing that stored energy to the crown of a tooth (or a group of teeth) and then over time, allowing that stored energy to dissipate as the springs elicit a physiological response from the living structures, evidenced by desirable tooth movement.
By design, orthodontic springs generate force levels that are regulated or throttled to fall within a range known to be safe and effective for initiating the physiological response of tooth movement. In response to gentle and continuous forces, the underlying bone supporting the roots of a tooth undergo a process involving the removal of bone (resorbtion) in the direction of movement of the root, along with the creation of new bone (osteogenesis) in the wake of root/tooth movement. This generally accepted model of tooth movement provides the basis for orthodontists to effectively reposition teeth according to a treatment plan.
In user the springs referenced above can take on a wide range of forms and configurations. Such springs can be formed from many materials ranging from latex and polymeric elastomeric materials, vacu-formed shells of olefin plastics and polycarbonate, to austenitic stainless steel, and other metallic alloys containing titanium, and cobalt as examples. Mechanical force generation can also be created by jack screws and other positive thread or magnetically-driven micro-mechanical devices. In those cases, the stored energy referenced above can be imparted directly to the skeletal structure of the face and skull, and in a sense, those boney structures themselves become the resistive spring. In those cases, the growth potential of young and rapidly growing adolescent patients can be desirably redirected to compensate for facial imbalances and anatomical deficiencies. For adults, application of gentle and continuous forces directly to bone can elicit other types of corrective boney responses. All of the efforts at correcting and reshaping malleable bone are generally referred to as bone remodeling.
As described above, the physiological responses allowing movement of a tooth's root through its boney support will occur only after forces failing within a range of biologically effective values are directed to the tooth. The osteo-based process of tooth movement involves complex cellular-level biological phenomena but those ideal forces, as well as forces above and below biologically ideal values can be categorized as subliminal, optimal or excessive, as follows.
Subliminal: When forces are below a minimum level, a tooth may exhibit slight repositioning as allowed by the tensing the periodontal fibers, but other than that, no true enduring, physiologically-based bodily movement of the tooth will be initiated. Such light forces are very unlikely to pose any concern of injury or trauma.
Optimal: When forces directed to the roots of the teeth fall within an ideal range, the osteogenetic processes proceed at a natural rate, without pathology or trauma to the surrounding living tissues/structures. The bioengineering of orthodontic armamentarium strives to deliver forces of this magnitude to effect correction. It is thought that the introduction of orthodontic forces triggers an otherwise natural balancing response of the living system and as such, forces within this range are unlikely to pose any concern of injury or trauma.
Excessive: Forces impinging on the tooth's root that exceed a maximum physiological force can cause injury to the surrounding tissues and the tooth itself. In such cases, any previously established rate of tooth movement will be markedly slowed and then it will stop. Excessive forces can result in patient pain stemming from such insult. Necrotic conditions within the periodontal socket, the periodontal membrane and the surrounding bone may result. In particular, the apical tip of the tooth's root may be blunted or resorbed, resulting in long axis shortening, a decrease in anchorage and other destructive long-term sequelae.
To quantify forces required for tooth movement, orthodontists have traditionally used an approach beginning with the determination of the area of the root of a tooth in plan-form as viewed perpendicular to the axis of movement. In other words, even though curved or cone shaped, and even though some teeth have multiple roots, a two-dimensional plan-form profile of the root structure is assessed and an estimate is made to arrive at a value for the area of root ensconced in bone. Next, a force constant will be established per unit area. For example, the multiple roots of a statistically average-sized lower first molar as viewed from the mesial may have a combined root plan-form profile of about 72 mm2. Using a value of 5 grams per mm2 as a physiological force constant, it can then be said that a distally-directed force of 360 grams (5×72) would constitute an ideal physiological force for moving a lower first molar of average size in the distal direction.
In practice, a rule of thumb value such as “360 grams to distalize a lower molar” will be established but an orthodontist may moderate that value somewhat lower for smaller than average-sized teeth or use a somewhat higher value than 360 grams for larger teeth. Further, the rule of thumb value may be further adjusted to accommodate patient age, sex and other holistic health factors. There is no practical need to precisely calculate exact root area or to precisely define target forces due to the fact that the living structure is tolerant of a relatively wide range of forces.
The example above involved the distal movement of a lower first molar. The same force constant value used for distal movement may be used for either distal or mesial movement and further, the force constant can be reliably used for all of the teeth being moved mesially or distally. Other force constants apply when considering the repositioning of roots in a lingual or labial/buccal direction. Similarly, still other force constants apply to the intrusion or extrusion of teeth. All of these considerations involve the initial step of estimating a realistic value for the plan-form area of the root as viewed along the axis of intended movement. Then the area resultant is multiplied by the appropriate force constant related to the intended direction of movement. In all cases, the objective is to determine the optimal force levels, on a tooth by tooth basis, to most efficiently move a tooth according to a treatment plan.
Even though the foregoing discussion covering methods for determining orthodontic force levels may generally represent traditional thinking in the field, many shortcomings and compromises are typically involved in the everyday delivery of orthodontic forces to teeth during orthodontic treatment. For example, during the early stages of treatment, the teeth and the arch slots of the brackets attached to them are chaotically positioned around both dental arches. As such, it will be determined that some of the teeth are positioned far from their desired finished positions and orientations while other teeth may be nearly in a correct or ideal position as is. Nonetheless, one continuous, monolithic archwire is normally installed in the bracket arch slots to begin the process of establishing order to all of the teeth. Such an archwire falls in the general class of orthodontic springs discussed earlier, where energy is stored as the spring is deflected. In an archwire's efforts to follow the lowest energy trajectory as it passes through each of the series of mal-positioned arch slots, varying degrees of archwire deflection and torsional twisting will be induced. Certain segments of the wire may exhibit high deflection, which can be associated with high levels of stored energy and conversely, segments of low archwire deflection store less energy. Teeth attached to the archwire within zones of high deflection will inherently receive significantly higher force levels and other teeth positioned within zones of low deflection will inherently receive much lower levels of corrective force. The thrust of this discussion is to convey the point that traditionally the levels of force delivered to any one tooth by an archwire is exponentially proportional to the extent to which the tooth is mal-positioned. Those archwire-generated forces are in no way proportional or in no way related to the area of the root supported by the underlying bone of the individual teeth that the archwire is attached to. It can be said that the archwire treats all brackets the same, regardless of the size of the tooth the bracket is attached to. Because of this, it is possible for small teeth to receive injuriously high corrective forces and for large teeth to receive ineffective, low forces from the archwire. In these instances, neither the large nor the small tooth will respond nearly as rapidly as when ideal forces are applied and in fact, those teeth may not move appreciably at all, and worse, the high force values associated with high archwire deflection may cause injury to the surrounding tissues as described earlier.
In practice, some skilled orthodontists use a number of methods to regulate the forces that are delivered to individual teeth. Additional wire length incorporated into the archwire, in the form of various types of loops and helices are sometimes installed to reduce the effective spring rate of the archwire. Accommodative bends intended to reduce the degree of deflection represent another means orthodontists use to differentiate forces, but such methods are by in large an art, directly dependant on the skill and craftsmanship of the individual doctor. Such steps can be called an art due to the fact that in practice, the net effect of such archwire modifications is seldom if ever quantified from a bioengineering standpoint and at most, represent a best guess on the part of the practitioner. Installation of such force-regulating features into the oral hardware, and the associated need for continual adjustment and activation of those features is time consuming and adds to the cost of treatment. Further, such force-regulating features invite breakage, can cause patient discomfort and can contribute to compromised hygiene. As a final comment on the use of force-regulating archwire features, current trends in orthodontics point away from the use of such features. Force regulation is more commonly relegated to optimizing the mechanical properties of the monolithic archwires themselves. Super-elastic (austenitic) nickel-titanium archwires exhibit a nearly flat stress/strain relationship and may represent an improvement over methods of the past, but use of such archwires has other reciprocal drawbacks, namely a very limited modulus of stiffness. They are very weak and can be effectively used only during certain phases of treatment.
To summarize the shortcomings of current methods, many of the springs used in orthodontic treatment exhibit an overly rapid spring rate. This is often due to space constraints in the mouth where space does not allow larger springs configured for a more desirable lower spring rate as described above. It is also due to the fact that lighter wire with a desirably lower spring rate tends to be more delicate and as such they are susceptible to distortion and breakage in the mouths of active and uncooperative adolescent patients.
Other problems triggered by the situation where an archwire exerts exponentially stiffer forces depending on the degree of mal-positioning exhibited by any one tooth can occur in a number of situations. Stiff, rapid rate springs that exhibit an exponentially increasing spring rate, such as those currently used in orthodontics may have a tendency to at first exert injuriously high forces before rapidly declining through the ideal force range and then dropping off to below a threshold force level, all within a matter of weeks or even days. This range from “too high to too low” progresses as a tooth responds to the forces and moves, and thereby unloads the driving spring. Orthodontic springs formed from latex or elastomeric polymers similarly tend to start out delivering high forces but then relax over the course of a day or two due to molecular slippage (creep) and water absorption. A gentle, constant and steady biological force is known to be ideal for tooth movement, but as can be appreciated from the foregoing, current practice of orthodontics involves forces that are often unknown and out of control. The unfortunate result is that a saw-tooth profile of forces are typically delivered to each tooth, where reactivation initiates a new cycle of high but then rapidly degrading forces. Such high, then low intermittent forces have been called “insult and repair” and as such are generally agreed by orthodontists as being undesirable and to be avoided to the extent possible. One reason intermittent forces are to be avoided is that the physiological processes of tooth movement, once initiated are best kept going. If allowed to stop, they are more difficult to get started again.
Solution to the Problem. The present invention addresses several of the shortcomings of the prior art in this field by providing an orthodontic aligner that employs a fluid-filled vesica to apply relatively uniform hydraulic pressure on the teeth within the optimal range of force, as described above. This approach helps to eliminate the problems associated with excessive forces or subliminal forces that have sometimes occurred with conventional orthodontic appliances.